The main progress made in the cytogenetic and molecular analysis of murine plasmacytomas and rat immunocytomas is as follows: (1)\the c-myc was mapped cytogenetically to the interface of bands D2 and D3 of chromosome 15; (2)\we have demonstrated the existence of translocation negative PCs that have an interstitial deletion in band D2/D3 of chromosome 15 PCs; (3)\we have identified PCs where the recipient (6;15) underwent a pericentric inversion leading to the same transposition and with the same breakpoints as previously observed in variant PCs where two separate chromosomes, 6 and 15, participated in the exchange; (4)\we have demonstrated the possibility to induce PCs in Balb/c backcross strains where the Balb/c derived chromosome 15 was replaced by CBA or AKR derived chromosome 15 indicating that there is no significant preference for the c-myc carrying chromosome 15 region in plasmatycomagenesis; (5)\we have shown close similarity in the translocation pattern of PCs induced in the Balb/c and NZB strains; (6)\the molecular analysis of the deletion PCs has revealed that the c-myc gene has been recombined with an alfa switch-like sequence flanked by DNA sequences showing no homology to the alfa-coding genes; (7)\in the rat, the c-myc gene has been assigned by Southern-transfer hybridization in the rat x mouse somatic hybrids to chromosome 7. The chromosome 7 is one of the chromosomes which takes part in the reciprocal 6/M translocation. (M)